220 2763 article Path: news.gmane.org!not-for-mail From: =?UTF-8?Q?Andrzej_Krzemie=C5=84ski?= Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: optional references -- take 2 Date: Wed, 6 Feb 2013 23:57:46 -0800 (PST) Lines: 127 Approved: news@gmane.org Message-ID: References: <8d57e9e9-e4cc-441c-abeb-e6526531fad5@isocpp.org> <3534ECE0-D67E-443E-9593-76B644FDDF6A@gmail.com> <3aca8f41-7a21-40d8-9bf9-9c27dfa778f3@isocpp.org> <5095aadf-58c1-458b-bc9f-6bbd7499aa59@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_1105_16985628.1360223866535" X-Trace: ger.gmane.org 1360223866 17028 80.91.229.3 (7 Feb 2013 07:57:46 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 7 Feb 2013 07:57:46 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDT2DGOJ34DBB654ZWEAKGQEY635NVA@isocpp.org Thu Feb 07 08:58:07 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-gh0-f198.google.com ([209.85.160.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1U3MMw-000898-Kg for gclcip-std-proposals@m.gmane.org; Thu, 07 Feb 2013 08:58:06 +0100 Original-Received: by mail-gh0-f198.google.com with SMTP id r11sf2983002ghr.9 for ; Wed, 06 Feb 2013 23:57:47 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-received:x-beenthere:x-received:date:from:to:message-id :in-reply-to:references:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:x-google-group-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=ppP9ngmmiTdAH+67ODyk+Wu6XPVDEuz+G7KhSyBRy8s=; b=NV6CXzmB6Ww2glTYvjCkWz/wgFykIbWYykw+rxasAt6oOXFZzzRaEXmZ3skE/tHf7R T3uoQV1xjifPNw+mXuyHhIqu2CSKNj/cKEQgnNlRV/FzgTL12n0VBY6gqIN7hQ0F8Szr LBBrdjoA9HtL24W5lEBnPQx487bC91QBZzPEfVPOscNJxht/6EmCuBe7z9mHrZTLkkm3 SwNvduz2ViYNGMJGbZkFoFKO18joCwsDz1ibw6XacQY5F//RXatssluR7Pl7GH4Mv1Z1 Ba0KyRzG04Sqx6k8rAAZ+Qeajb1UKT4yuWZpF2DHFPPP2rDd+zAK7H3nsDkrW9dJhQ9Q y/eg== X-Received: by 10.224.178.207 with SMTP id bn15mr658312qab.1.1360223867298; Wed, 06 Feb 2013 23:57:47 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.16.37 with SMTP id c5ls601862qed.80.gmail; Wed, 06 Feb 2013 23:57:46 -0800 (PST) X-Received: by 10.49.75.9 with SMTP id y9mr23177qev.9.1360223866921; Wed, 06 Feb 2013 23:57:46 -0800 (PST) In-Reply-To: <5095aadf-58c1-458b-bc9f-6bbd7499aa59@isocpp.org> X-Original-Sender: akrzemi1@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:2763 Archived-At: ------=_Part_1105_16985628.1360223866535 Content-Type: text/plain; charset=ISO-8859-2 Content-Transfer-Encoding: quoted-printable W dniu czwartek, 7 lutego 2013 04:55:22 UTC+1 u=BFytkownik Nicol Bolas=20 napisa=B3: > > > > On Wednesday, February 6, 2013 7:25:09 PM UTC-8, Nevin ":-)" Liber wrote: >> >> On 6 February 2013 20:40, Nicol Bolas wrote: >> >>> That's really the question I have about this whole `optional` thing= :=20 >>> what exactly is the use case for this functionality that `T*` can't als= o=20 >>> cover? >>> >> >> How about optional? >> > > I would say that should be illegal. Consider: > > T&& t1 =3D GetT(); > T t2(t1); //Calls copy constructor. > T t3(std::move(t1)); //Explicit std::move required to cause move. > > optional ot1 =3D GetOptT(); > T t2(*ot1); //Calls move constructor. No explicit std::move required. > > Much language was expended in the C++11 standard to ensure that you *must= *use an explicit std::move for anything that isn't a temporary or the retur= n=20 > from a variable that's going out of scope. optional breaks this,=20 > allowing movement without the explicit use of std::move. > > optionalis dangerous. > And it is explicitly banned in the proposal.=20 --=20 ---=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/?hl=3Den. ------=_Part_1105_16985628.1360223866535 Content-Type: text/html; charset=ISO-8859-2 Content-Transfer-Encoding: quoted-printable

W dniu czwartek, 7 lutego 2013 04:55:22 UTC+1 u=BFytkownik Nicol Bo= las napisa=B3:


On Wedne= sday, February 6, 2013 7:25:09 PM UTC-8, Nevin ":-)" Liber wrote:
On 6 February 2013= 20:40, Nicol Bolas <jmck...@gmail.com> wrote:
That's really the question I have about this whole `optional<T&= >` thing: what exactly is the use case for this functionality that `T*` = can't also cover?

How about optional<T&&>?

I would say that should be illegal. Consider:

T&& t1 =3D GetT();
T t2(t1); //Calls copy constructor.
T t3(std::move(t1)); //Explicit std::move required to cau= se move.

optional<T&&> ot1 =3D GetOptT();
T t2(*ot1); //Calls mov= e constructor. No explicit std::move required.

Much language was expended in the C+= +11 standard to ensure that you must use an explicit std::move for a= nything that isn't a temporary or the return from a variable that's going o= ut of scope. optional<T&&> breaks this, allowing movement wit= hout the explicit use of std::move.
optional<T&&>is = dangerous.

And it is explicitly banned in the prop= osal.

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